1. Field of the Invention
The present invention relates to an organic light emitting diode, and more particularly, to an organic light emitting diode using edge emission and having long lifespan.
2. Description of the Related Art
Generally, organic light emitting displays include an organic light emitting diode (OLED) laminated on a thin film transistor (TFT) in order to form an electronic circuit so that light is emitted in response to a signal controlled by the TFT. The organic light emitting diode includes a light emitting part laminated between an anode and a cathode. When a current flows through the organic light emitting diode, the light emitting part is excited by combining holes with electrons (i.e., electric coupling), so that energy is emitted in the form of light. The light emitting part includes a hole injecting layer (HIL), a hole transporting layer (HTL), and a light emitting layer (EML).
Light is not uniformly generated throughout the light emitting layer. Specifically, the light emitting area is concentrated at the interface between the hole transporting layer and the light emitting layer. When the laminated light emitting part includes a hole injecting layer, a hole transporting layer, and a light emitting layer sandwiched between a cathode and the anode, light emission is concentrated essentially only at the interface between the hole transporting layer and the light emitting layer since electrons migrating from the cathode are concentrated at this area.
In the meantime, when the organic light emitting diode is used extensively, the material property of the light emitting part changes, which leads to an increase of the electrical resistance thereof. Thus, when an insufficient current flows through the light emitting part because of such an increase in resistance, light emission decreases. Therefore, the luminance of the organic light emitting diode is reduced.
This deterioration in light emission over time occurs especially when the light emitting layer is made out of a polymeric material. Specifically, to form such a light emitting layer, a polymeric material is first dissolved in a solvent, and thereafter the layer is formed using a coating method such as spin coating. When the light emitting layer deteriorates over time, the property of the dissolved polymeric material is changed and the polymeric material no longer remains dissolved and produces an insoluble layer. Thus, the material property of the light emitting layer becomes identical to that of the insoluble layer, and the resistance of light emitting layer is increased, leading to decreased light emission.
The luminance of the organic light emitting diode rapidly decreases with time and drops to below 50% of the initial value after approximately 110 hours of use. This reduction in the luminance is referred to as “a tail drop”, and is caused by the increase of the resistance of the polymeric material of the light emitting layer, as described above.
When a voltage is applied to the organic light emitting diode in the case where the luminance drops below 50% of its original value, the center of the light emitting layer becomes dark and the edge thereof becomes relatively bright. This phenomenon is called “edge light emission” and it is caused because the current flowing through the light emitting layer avoids the insoluble area at the center where the resistance has increased. That is, when the light emitting layer becomes insoluble, the resistance thereof increases, and current does not flow through the light emitting layer and thus light emission is not generated. Normally, when the resistance of the light emitting layer is low and before it rises, the current flows through the light emitting layer along the shortest possible path so that the light emission is generated at the center of the organic light emitting diode. However, after the organic light emitting is extensively used, the resistance of the light emitting layer rapidly increases at the center thereof so that the current flows at the edges of the light emitting layer where the resistance is relatively low. Thus, although light emission is generated at the edge of the of the light emitting layer upon tail drop, the luminance at the center of organic light emitting the diode is below 50% of its original value, so that at this point in time, it is considered that the lifespan of the organic light emitting diode as a display element has ended. Therefore, what is needed is an organic light emitting diode with increased lifespan by increasing the area in the light emitting layer that can emit light during later stages in the life of the OLED.